This project is concerned with analysis of the developmental characteristics of gonadotropin receptors during fetal life and sexual maturation. Studies are performed on the emergence of receptors and biochemical responses of the pituitary-gonadal system during development, on the maturation of testicular and ovarian endocrine function, and on the control of gonadal sensitivity during sexual matuaration. The current studies have defined the changes in sensitivity to gonadotropin stimulation that occur in the rat Leydig cell during early development. These changes include a progressive loss of the trophic response to gonadotropins, with acquisition of the adult-type response after about 20 days of age, the latter being characterized by a period of refractoriness after the initial stimulation of Leydig-cell function. This temporary phase of testicular desensitization is ude to loss of LH receptors and lesions of the steroidogenic pathway which are in part attributable to an estrogen-mediated cnsequence of increased audrogen biosynthesis. The absence of these changes in the fetaland neonatal testis may be related to the lack of detectable estrogen receptors in the first several days of life, and to differences in the rates of turnover of membrane receptors between the fetaland adult Leydig cell populations.